Passive sensors of the surface acoustic wave (SAW) type can be interrogated remotely. In practice, at least one sensor is embedded onboard a support disposed in the environment of the measurement to be performed. A remotely placed emission and reception system dispatches an electromagnetic wave toward the sensor coupled to a reception antenna. The emitted wave is picked up by this antenna and the sensor enters a forced regime. When this wave is cut off, the sensor continues to oscillate with its natural frequency and emits, via its antenna, a damped sinusoidal signal of very low power, which is thereafter detected by the emission reception system. The natural frequency, or resonant frequency, of the sensor being dependent on the physical quantity to be measured, the determination of this frequency makes it possible to perform this measurement.
Currently used procedures are based on an emission system employing a wave train, either fixed frequency, or broad spectrum. The weak signal originating from the sensor and received by the system reception and processing unit is transformed by frequency mixing often by two oscillators in quadrature into very low or zero intermediate frequencies and a demodulation notably of “I & Q” type, usually entirely digital, enables the resonant frequency Fo of the SAW sensor, and therefore the value sought, to be obtained approximately. In the current state of the art, to have sufficient accuracy in the determination of Fo demands numerous successive measurements and their processing generally requires several tens of milliseconds.
Moreover, this type of processing, generally based on a statistical study of the results of supposedly identical successive measurements, makes it imperative not to have large variations in the measurement conditions during the tens of milliseconds required for the measurement. These variations may notably be due to the evolution of the position or the speed of the sensors, the fast evolution of the conditions of propagation of the emitted and re-emitted waves or else the fast evolution of the quantity to be measured, these evolutions being significant when the duration of the measurements exceeds a millisecond.
It follows that these measurement systems do not make it possible, in the customary measuring conditions, to carry out a reliable and/or accurate measurement.